I've got the simplicity 9523 and my dad has the AC 5020. If you look on tractor data I think you will find the AC 5030 is listed at 28HP whereas ours are listed at 25HP. As stated, I would assume engines would swap. Not sure the hours you have on yours but I have seen these at 4000 + hours. Mine has 580ish hours. My dads has 1400ish hours. Both run good. I hope both continue to run good cause this particular tractor is hard to find engine parts for. Fortunately, there were like 5000 of them produced under the AC logo alone I think not mention simplicity, hinomoto, Massey Furgeson too. So they are out there. These are nice midsized tractors for those OK with 2wd. Although there was a 4wd version but more rare as I suppose few really saw the need for it back in the mid to late 70's. Put some chains on and go you know! They have a great and tight turning radius compared to many tractors so using a MMM deck to cut grass is great.
Anyways. Have you contacted a company like Falicon in Florida to inquire about this situation? If the oem rods are $500 each then it seems to me you could probably get custom made rods or something very close out of another application if you have your specs. But maybe not as this day and age it seems few have the talent to make these things happen and the few who do are so expensive that making it happen makes no sense at all unless sentimental issues involved.
There are many of these tractors running well and happy owners of many years. We just picked these two up last year so hoping for good luck.
How many hours were on your tractor? Seems strange to me it would just all of a sudden start making loud clunking and wear out those rods. I don't think that happened in a minute.... ?? Were there unusual noises in the motor before this?